Due to problems of environmental pollution, there is increased interest in the separation of oil from mixtures of oil and water. Such separations may be necessary in large bodies of water, as in the case of oil slicks on oceans and lakes caused by leakage from tankers, drilling rigs, and storage tanks, which cause serious pollution of both open water and to the shore, and consequently injury to aquatic and terrestrial fauna and flora. Similarly, oil-water mixtures resulting from industrial activities has produced serious pollution problems when discharged into rivers, streams and other bodies of water. For plants with central wastewater treatment systems, the discharge of large volumes of oily waste water is an expensive and difficult treatment burden.
For example, many machine parts or heat treated parts are washed in parts washing tanks, and the washing solution is contaminated with manufacturing oils and heat treating quench oil. In the past, this contaminated wash water has been discharged into the sewage system, but due to the resulting pollution of the water system with oil, this practice has been discontinued; as a result, it became necessary to haul the oil-contaminated wash water away to disposal sites or for further separation treatment.
Other factors engendering increased interest in the separation of mixtures of oil and water include economic considerations, and work place safety and health. Oily contamination in the parts washing solutions contributes to an inefficient cleaning process which typically requires secondary cleaning and manufacturing steps to correct. These subsequent steps present added expense and time requirements to the manufacturing process. Environmentally, the contaminating oil in parts washers is often carried into subsequent heat treating tempering furnaces where the oil burns off as smoke in the plant and smoke discharged from the plant's smokestacks to the local environment. The health and safety of the workers is also damaged by the presence of this oily contaminant. Not only is the in-plant smoke a danger, but frequent changes of washing baths due to oil contamination require increased worker exposures to cleaning fluids and related handling hazards.
Several types of treatment methods and systems have been developed in efforts to efficiently separate oil from water-oil mixtures. One treatment method is filtration, by which oil is entrapped by a filter. Filtration may be accomplished by barrier filters, which include bag and cartridge filters, or by membrane filters, which filter fluids down to minute dimensions and are typically designed to remove emulsified oil from water. However, filters tend to clog quickly, and are time consuming and expensive to replace.
Another treatment method depends upon the use of gravity to separate a mixture of oil and water. Gravity separation exploits the difference in specific gravity between oil and water. A gravity separator typically consists of a large holding tank, in which oil rises to the surface of the water. These tanks must be substantially turbulent free to operate, and require an additional means with which to remove the oil. Thus, the tanks are filled, the oil collects at the surface and is removed, and the water returned for reuse or discharged. These tanks are slow, and require very large surface areas. Improved gravity separation involves horizontal separators. In these separators, water enters at one side of a horizontal tank, and as it flows to the other side, the oil rises to the surface, where it can be collected. The cleaner water is then discharged from another side of the tank. The oil is typically collected by means of a baffle, which holds the oil-water interface below the top of the baffle; the floating oil which accumulates above the interface then spills over the top and into a separate compartment from which it can be collected and discharged.
The use of coalescers in horizontal separators is well known. Coalescers are generally tightly packed beds of coalescing media or closely spaced plates, which aid in the separation of oil from water. Typical coalescer configurations are stacks of closely spaced plates, angled from vertical to horizontal. The plates may also be grooved or channeled, or wavy. Under the influence of gravity, oil separates from an oil/water mixture at a rate determined by Stokes law. This formula predicts how fast an object will rise or fall through a heavier fluid based on the density and size of the object and the distance it must travel. In a packed media bed coalescer, oil is exposed to large amounts of surface area provided by the coalescing media. As the oil-water mixture passes through this media, oil droplets are temporarily held by the coalescing media where they are exposed to further contact with oil molecules in the mixture. This physical contact on the surface of the coalescer media has the effect of increasing or coalescing the size of the oil droplets in the mixture. In closely spaced plate and corrugated coalescing separators oil rises only a short distance where it is captured on the underside of the coalescing plates. While the use of coalescers improves the performance of horizontal separators, the coalescers are very susceptible to clogging. In operation, these coalescing horizontal separators have the same type of failure as do filters, in that the coalescers can quickly clog and become blocked, thus requiring frequent and expensive maintenance. In addition, these coalescing separators still require a large footprint.
Another type of gravity separation is achieved by vertical separators. These separators generally involve discharge of an oil-water mixture into a vertical tube, which is generally open at the bottom and which sits in and thus empties into a water environment. This environment may be an open body of water, such as a lake or ocean, or it may be a collecting tank. The mixture is discharged near either the upper end or the lower end of the tube. As the mixture flows into the tube, the oil rises and the water sinks, effecting separation of the two different fluids. The cleaner water is discharged from the bottom of the tube into the surrounding water, whereas the oil collects at the top of the tube. The oil may be collected be means of a tube and a pump, or it may be discharged by means of an overflow tube.
A common use of such vertical separators is in off shore waste water separators. One type is a submerged caisson, which is a large diameter pipe projected vertically downward into the water and open at the bottom. A small diameter pipe is inserted vertically downward inside the large caisson to about two-thirds of its length. Oily waste water drains into the smaller interior pipe and flows downward and out into the large caisson, whereupon the flow of the water proceeds at a much lower rate, due to the larger diameter of the caisson. This allows oil droplets to separate and rise up to the surface, and the clarified water to flow down to and out the open bottom. The collected oil is allowed to accumulate until the oil-water interface is just above the inlet pipe, at which point the inflow of water is halted, and the oil pumped out. A major problem with this type of vertical separator is that the discharged, or effluent, water, still contains a considerable amount of residual oil; typically, the oil content of the water effluent is about 20% of the total initial amount of oil in the waste water.
Coalescers have also been used with vertical separators. One of the simplest is the use of gas bubbles in a pile skimmer, which is similar to the submerged caisson described above. In this system, the oily water mixture is introduced near the middle of the pipe, and gas is injected near the bottom of the pipe. The gas or dispersed gas bubbles contact and attach themselves to the oil droplets in the water, thus enhancing the gravitational separation by flotation. In another and more complex separator, a vertical tank contains a plurality of inclined corrugated plates. An oil-water mixture is introduced near the bottom of the tank, and then flows upward under pressure through the corrugated plates, which improve coalescence of the oil. The oil is directed upward by an oil channel, and the clarified water is then downward to a clean water outlet.
While these vertical separators represent improvements, they also possess distinct disadvantages. For example, open bottom cassions or skim piles do not manipulate the flow of the mixture to be separated in any way to effect physical separation between the oil and water, except by slowing down the inlet feed as it enters the separator. Due to this coarse separation mode, cassion type separators require large diameters (footprint) and large internal fluid volumes to create the separation conditions required. Thus, they are most suitable for open bodies of water; moreover, the recovered oil is not very dry, and a high percentage of the initial oil remains as residual oil in the clarified discharged water. The addition of counterflow compressed gasses requires the expense and maintenance of expensive air compressors and related equipment. These open bottom cassions or skim piles also require a substantially closed upper end to effect oil capture, which has the effect of making maintenance more difficult and making operational adjustments more difficult to visualize and calibrate accurately.
It would be very useful to provide a vertical oil-water separator which has no moving parts and which is open to the atmosphere, which is low maintenance and easy to operate, which can be operated in-line as in an industrial setting, and which has a very small footprint and is easy to install.